Why is QBO using an invoice number I've already used, and how can I change it?

Hello there, @DBarrow2.

I want to ensure you can change the number sequence for your invoice. 

QuickBooks Online assigns numbers to transactions by default. To check if custom transaction numbers are enabled, click the Settings button at the top right of your invoices and tick the Custom transaction numbers checkbox.
 

Once done, let's create a new invoice or locate an existing one and change its number by following these steps:

1. On the left pane, select Sales, then click Invoices.
2. Choose any invoice, then select Edit invoice.
3. In the Invoice no. field, enter the custom number or letter sequence you want to use for your forms.
4. Tap Save and close.

See these resources to learn more about setting your own numbering preference and personalizing invoices:

• Add custom transaction numbers to sales forms in QuickBooks Online
• Customize invoices, estimates, and sales receipts in QuickBooks Online

Once done, check out this article for the complete instructions on receiving and processing payments: Record invoice payments in QuickBooks Online.

Don't hesitate to get back here if you have any other concerns about managing your invoices in QuickBooks, @DBarrow2. Have a good one and keep safe. 

Thank you for the help in changing the numbering. Is there a way to set the numbering so QBO doesn't automatically assign an invoice number which has already been used?

Thank you for your getting back to us, @DBarrow2. 

QuickBooks Online (QBO) generates invoice sequence numbers from 1001 by default, but you can always change the invoice number by your preference. If you have already turned on the Custom transaction numbers checkbox, the transaction numbers will follow the sequence in which you create them. For example, it could start with AB1001.

Furthermore, QBO automatically detects if there's an existing invoice using a particular number. If you use it again, QBO will reset the invoice numbers. Therefore, there's no direct way to stop the program from assigning invoice numbers already used once they select Yes from the prompt but to change the Invoice no. per invoice manually.

I'd like to provide you with this article to learn more about invoice numbers: How to assign invoice numbers: Definition & examples.

Furthermore, you can also create recurring invoice transactions to schedule an invoice and save more time.

You can always comment below if you have further questions besides invoice numbers in QuickBooks Online. I'm here to assist you. Have a great day!